I think there is something on the tree that is not ornament...can you see it?
Last year my two black cats Mo & Dez had their first Christmas and I'm convinced its their favorite Holiday. The minute we brought that tree home both of them jumped right in it and attacked the ornaments as if they were birds. My cats are partial outdoor ones so they must have thought, hey its a tree, lets jump all over it and eat the moving things on it. Have your cats ever done this before? It was so strange and funny to see them act like little hunters.